The present invention relates generally to methods and alloys for adding to steel machinability increasing ingredients and more particularly to a method or alloy for adding lead and bismuth to steel.
Lead and bismuth enhance the machinability of steel. It is desirable to add lead and bismuth to steel together, e.g. as a lead-bismuth alloy, because this improves the uniformity with which the lead and bismuth are distributed in the steel.
Both lead and bismuth have relatively low melting points, lead having a melting point of 327.degree. C. (621.degree. F.) and bismuth having a melting point of 271.degree. C. (520.degree. F.). When lead and bismuth are combined together in an alloy of the two, the resulting alloy has a melting point even lower than that of its constituents. For example, a lead bismuth eutectic (55.5% bismuth and the balance lead) has a melting point of about 125.degree. C. (257.degree. F.). Because a lead-bismuth alloy has such a low melting point, problems will arise when this alloy has been introduced into steel. For example, because of the low melting point, the lead-bismuth alloy may separate to the bottom of an ingot mold into which molten steel containing the lead-bismuth alloy has been poured for casting into an ingot. Moreover, during hot rolling of the steel, the lead bismuth alloy may be squeezed out of the steel shape undergoing hot rolling.